Microbiology Case Study: A 51-Year-Old Woman with a Tick Bite

Case History

A 51-year-old female with a past medical history of chronic lower back pain, depression, and anxiety presented to an outpatient dermatology clinic for a lesion on her left cheek that was present for years but has recently grown and become irritated. Additionally, she reported a “skin taggy thing” that she first noticed on her posterior neck about two days ago. Upon physical examination, a tick was observed latched onto the right posterior neck. After being alerted to the presence of a tick, the patient did disclose that she was in the woods three days prior. The bite site was locally anaesthetized and the tick was removed and sent to the laboratory for arthropod identification. Furthermore, the patient received a single prophylactic dose of doxycycline 200 mg.

Ixodes scapularis (black-legged ticks), also known as deer ticks (their preferred hosts are white-tailed deer), are small arachnids. As obligate ectoparasites of vertebral hosts, I. scapularis are also willing to feed on humans. Importantly, infected arthropods can be vectors of multiple pathogens including: the spirochete, Borrelia burgdorferi, that causes Lyme disease; the intracellular gram-negative bacterium, Anaplasma phagocytophilum, that causes human granulocytic anaplasmosis; the Apicomplexan parasites, Babesia spp, that cause babesiosis; and the flavivirus, Powassan virus, that causes encephalitis.

The I. scapularis life cycle, ranging from one to two years in length, is composed of four developmental stages: egg, larva, nymph, and sexually dimorphic adult. Compared to nymphs and adults that have eight legs, larvae are smaller and have six legs. The term “three-host cycle” implies that during each of the three motile stages, I. scapularis takes a blood meal from a different host animal, at which time the tick’s saliva is injected and transmission of pathogens can occur.

Hard ticks possess an anterior capitulum (Image 1, A) whereas soft ticks lack a capitulum. The capitulum is made up of mouth parts that are attached to the basis capituli. The mouth parts refer to paired appendages called palpi (Image 1, B) that are parallel to a median hypostome (Image 1, C). The hypostome holds teeth-like structures, called denticles, arranged in a specific formula useful for identification. The mouth parts (palpi and hypostome) are longer than the width of the basis capituli (Image 1, D) and this ratio is also useful in identification. The dorsal surface of the basis capituli has two porose areas (Image 1, E) in adult females that secrete wax to waterproof eggs. The dorsal shield, called a scutum (Image 1, F), is absent in soft ticks and inornate compared to other hard ticks. In adult males, the scutum covers nearly the entire dorsum. Other hard ticks have eyes (lateral markings on the scutum) and festoons (grooved bulges on the posterior margin) that are both absent in I. scapularis. The ventral surface demonstrates coxa (Image 2, G) that are the basal segments of paired legs, numbered I-IV from anterior to posterior. Posterior to coxa IV are paired spiracular plates (Image 2, I), external openings of the respiratory system. A median genital aperture (Image 2, H) is present in adults. The distinct anal groove (Image 2, J) is an inverted U-shaped curve located anterior to the anus (Image 2, K) in all Ixodes species, as opposed to posterior or indistinct anal grooves of other genera of hard ticks.

Following arthropod identification in parasitology, microbiology laboratory reports include: the genus (Ixodes), species (scapularis), developmental stage (adult female), level of engorgement (unengorged), and status of mouth parts (partially intact). The genus and species are of medical importance because of their characteristic associations with various human pathogens. Of the developmental stages, nymphs and adults are most frequently associated with human transmission of A. phagocytophilum. Also, larvae are unable to transmit B. burgdorferi because the spirochete is not transmitted vertically to eggs; as such, a blood meal from a reservoir host is required. Nymphs, being smaller in size than adults, are more likely to complete feeding undetected and thereby transmit B. burgdorferi. Feeding is necessary for adult females to achieve fertility; therefore males are less likely to be discovered on hosts. The level of engorgement, an estimate of feeding time, is relevant because approximately 36 hours are required for B. burgdorferi to multiply in the tick’s midgut and migrate to salivary glands for transmission to a host, by which time ticks are visibly engorged. Lastly, if the mouth parts (palpi and hypostome) are intact, this suggests that there was only brief host contact and it is less likely that pathogen transmission occurred.